1. Field of the Invention
The present invention relates to the field of the clamping of bolts by cold prestressing and, in particular, to a method for controlling tensile stress of a bolt shank, such as a screw or dowel pin, in order to check whether the shank is sufficiently prestressed at the end of clamping. The invention also relates to a device for carrying out said method.
The deformation energy of the bolt is transferred to the nut bearing on the structure to be assembled. The components are thus held in place with clamping.
2. Description of the Relevant Art
Bolts, comprising a shank, such as a screw or pin, and one or more nuts, are used for assembling a plurality of components of a structure with clamping. It is known, in order to obtain high-quality clamping, to carry out clamping by tensile prestress, in which an extension of a clamping rod is performed with the aid of an actuator, for example a hydraulic jack, then a clamping nut is brought into contact, without a high torque being exerted on a face of the structure to be assembled, and subsequently the extension force on the rod is released. The released rod tends to recover its length at rest, but is blocked by the nut. The deformation energy imparted to the rod during the extension step is transferred to the rod/nut assembly bearing on the structure to be assembled. The components are thus held in place with clamping or prestress.
Such a method avoids tightening a nut by applying a screwing torque to it. This affords higher accuracy in the actual clamping force and has the advantage, on the one hand, of not causing any friction under load of the components to be clamped and therefore of avoiding damaging the surfaces of the components, and, on the other hand, of not generating torsion in the body of the rod, thus reducing the level of equivalent stress in the body of the rod and, moreover, eliminating the risks of inopportune unclamping in the event of vibration. Such a method is highly suited to the clamping of any material, for example alloy steels conventionally used in bolt manufacture, but also stainless steels, titanium or composite materials which are highly sensitive to friction and to seizure and/or torsion.
Once clamping has been carried out, however, it is necessary to ensure that the residual tensile force of the rod, that is to say the prestressing force of the rod which is applied to the structure to be assembled, is sufficient.
A simple control method involves measuring only the tensile or extension force applied to the rod by the actuator in order to bring the nut into contact, and subsequently, depending on the particular application, extrapolating the value of the final residual clamping force obtained after the release of the extension force. This extrapolation takes place generally by means of a graph or with the aid of experimental data. Nevertheless, such a control method does not afford satisfactory accuracy for many applications. It is often necessary, before clamping on site, to conduct tests under conditions similar to those of the application in question, this being especially costly. Furthermore, because of the low accuracy, a margin of error is generally adopted, which usually leads to the provision of oversized bolts.
Another method may be to determine the residual elongation of the rod after release. For this purpose, the residual elongation of the rod is measured before and after clamping by means of a displacement measuring device, such as a tracer-type sensor, for example a comparator or inductive sensor, or an ultrasonic device. Where the comparator or inductive sensor is concerned, it is generally necessary to have a reference gage placed in an axial passage machined at the center of the clamping rod, thus incurring high machining and manufacturing costs.
The document FR-A-2 586 098 describes a device for controlling the tensile elongation of a pin with the aid of a gage, comprising a tracer-type sensor arranged on the adjacent ends of a reference gage and of a pin, the outputs of the tracers being connected to a processing unit, in which an elongation value and a nominal pressure value not to be exceeded are entered.
The measurement of the length of the clamping rod may be carried out alternatively, as mentioned above, with the aid of an ultrasonic sensor measuring the time taken by an ultrasonic wave to execute or more transits between the two ends of the rod. This method requires the use of ultrasonic measuring equipment which is generally highly costly.
These control methods have a disadvantage of requiring prior calibration. Calibration involves determining, for each type of rod of a specific application, the correspondence between the elongation of the part under clamping and the tensile force in the rod. This may prove particularly lengthy and costly in the case of applications employing a large number of clamping rods and where the high level of accuracy required makes it necessary to calibrate each rod which has to be labeled in this way. A practical example is the clamping of nuclear reactor vessels.
Moreover, calibration must be carried out in a laboratory on high-precision traction machines which are costly to put into operation. Furthermore, it is necessary to ensure that the calibration conditions are identical or similar to the clamping conditions in the application in question, so that calibration is relevant.
It will also be noted that the use of sophisticated measurement means, such as ultrasonic sensors or inductive sensors, increases the cost of carrying out the method. This is especially true in applications where there is a plurality of rods which have to be controlled simultaneously.
Finally, with the correspondence between residual elongation and clamping force being defined, it still remains to ascertain, for the particular application, what extension force will have to be applied in order to make it possible to achieve the residual force required for clamping.